The present invention relates to a method and apparatus for determining angular displacement of one or more parts of an object, said method comprising directing at least one beam of electromagnetic radiation towards one or more parts of the object, detecting speckles from the object surface in at least one first image sensor, and determining one or more speckle displacements before and after the angular displacements of said one or more parts of the object as a function of the angular displacements.
The invention also relates to a method of determining twist of a rotating shaft subjected to a load; a method of simultaneously determining angular displacements and surface translations of one or more parts of an object; and a method of determining the radius of angular displacement of an object.
1. The Technical Field
Generally, determination of angular displacement of an object using speckle displacement comprises illumination of the surface of the object by a laser beam. The reflected laser beam generates a speckle pattern which is detected in a linear image sensor. After angular displacement of the object, the displaced speckle pattern is detected in the linear image sensor. The two speckle patterns are then cross-correlated to determine the angular displacement which is provided by the displacement giving maximum cross-correlation.
A drawback of this method is that the determination of the angular displacement depends on the distance from the linear sensor to the object, movements of the object, the radius of displacement, as well as the shape of the object.
2. Prior Art Disclosures
Hayashi and Kitagawa, "High Resolution Rotation-Angle Measurement of a Cylinder Using Speckle Displacement", Applied Optics, Vol. 22, 1983, p. 3520-3525, disclose a method for measuring the rotation angle of a cylinder, particularly a small rotation angle with high resolution based on the speckle displacement detection in the (near) diffraction field caused by rotation of the cylinder surface. The light intensity distributions of speckles before and after the cylinder surface displacement are detected by a detector array, and displacement of the speckle pattern is measured by computing their cross-correlation function.
Yamaguchi and Fujita, "Laser Speckle Rotary Encoder", Applied Optics, Vol. 28, No. 20, 1989, p. 4401-4406, disclose a method and apparatus for measuring small rotation angles of a cylinder by detecting displacement of the speckles which appears in the diffraction field of the surface illuminated by a narrow laser beam. The speckle pattern is detected by a linear sensor whose output is cross-correlated before and after the rotation of the cylinder. A resolution of 2 millidegrees is reported.
In both of these prior art techniques, the angular displacement measurement is dependent of the distance of the linear sensor to the cylinder being measured, longitudinal and transversal movements of the cylinder, its radius, and the shape of the object.